Traditionally, pressurized aircraft use Environmental Control Systems (“ECS”) to maintain cabin pressurization and to control cabin temperatures during flight. In order to maintain cabin pressurization and temperature, outside air is supplied to the cabin via air conditioning packs and a portion of the air in the cabin is recirculated by recirculation fans to provide an acceptable level of volumetric airflow to the aircraft passengers. In many cases, recirculation fans run at a constant speed and the total volumetric airflow to the cabin is maintained constant by modulating the flow provided by the air conditioning packs. In some case, recirculated air is passed through a filter to improve cabin air quality. In other cases, aircraft carry cabin humidifiers and/or dehumidifiers to further condition cabin air.
As the amount of outside air provided by the packs increases, aircraft fuel efficiency is reduced. This is especially true when the air conditioning packs use bleed air from the aircraft engines. Accordingly, it can be desirable to reduce the amount of outside air flowing into the cabin via the air conditioning packs.
In some cases, aircraft manufacturers have designed aircraft to provide a selected amount of outside air to the cabin based on the number of cabin occupants. In this case, a crew member manually enters the number of passengers into a control panel connected to the ECS system. Accordingly, when the aircraft carries fewer passengers, the amount of outside air that enters the cabin can be reduced. In other cases, the ECS system is configured to provide a selected amount of outside air to the cabin based on the maximum number of passengers that can be carried by the aircraft. Additionally, in some cases the recirculation flow rate can also be adjusted using valves and/or flow restrictors to reduce the flow rate provided by the constant speed recirculation fans. As fuel cost increase, it can be important to provide good cabin air quality in a fuel efficient manner.